Carbon steel structures, including steel pipelines, are often provided with protective coatings to prevent corrosion. Most protective coatings applied to carbon or similar low alloy steels form a physical barrier between the corrosive environment and the steel surface. The performance of coatings for steel surfaces varies greatly due to the chemistry of the different resins used for coating materials, variations in type of pigment, coating thickness and surface preparation of the steel substrate prior to the coating process. A common failure mode of polymer type coatings, in particular, is the loss of adhesion between the steel surface and the coating itself. Often, the adhesion loss is so complete that the coating will disbond entirely from the steel surface in the form of blisters. Of course, once a blister has formed, water and gas can usually penetrate the coating, gather behind the blister and cause substantial corrosion to the steel.
Certain coatings are applied to steel surfaces over so-called primer coats, at least some of which have better adhesion to steel than conventional polymer type topcoats. The primer coat is intended to provide a more favorable substrate surface to which the top coat or primary coating can bond. However, primer coats have the same disadvantages as most conventional polymer coatings, namely the relatively weak adhesion between the steel and the primer coat layer.
The bonding of polymer-type coatings, including primer coats, to a steel surface is primarily driven by weak Van der Waals forces and limited dipolar attraction from some of the radical groups on the polymer backbone. Even freshly cleaned steel has a thin oxide layer on its surface wherein a polymer type coating must first wet the steel surface completely, then dry and/or crosslink while maintaining good contact with the steel surface. Although there is some mechanical bonding between polymer coatings and steel surfaces, depending on the steel surface profile, when a force stronger than these bonding forces acts on the interface, the coating can fracture, delaminate or, most commonly, break the interfacial bond in the form of a blister.
The conventional solution to preventing blistering of polymer coatings to steel surfaces has been to increase the strength of the bond by increasing the bonding area. This is commonly carried out by mechanical abrasion of the steel surface such as sand blasting to increase the bonding surface area. Certainly, surface cleanliness is important and removal of all contaminants such as dirt, oil, and previous corrosion from the steel surface must be accomplished to improve adhesion between polymer coatings and carbon steel surfaces, in particular.
One environment wherein a high degree of surface cleanliness is difficult to achieve is in situ coating of steel pipelines used in the oil and gas industry, for example. Efforts to coat the interior surfaces of steel pipelines which have been conducting hydrocarbon materials, such as crude oil and refined petroleum products, have had only moderate success. The cleaning process for a pipeline which has been in service for any period of time requires removal of all of the pipeline transport product from the line, removal of residues such as paraffins, asphaltenes, and waxes from the pipeline surfaces and removal of any previous corrosion product (rust) from the steel surface prior to application of the coating. Complete removal of all of these materials is time consuming and difficult to accomplish. Moreover, preparation of the steel surface to increase the bonding area, such as by sandblasting, is also difficult to accomplish and time consuming when being carried out in situ on existing fluid transport pipelines.
Accordingly, there has been a strong need to develop improved coating adhesion to steel surfaces, including, in particular, the interior surfaces of new and existing steel pipelines used for transporting fluids in the oil and gas industry. It is to these ends that the present invention has been developed.